The Question:

Why do we say some foods are healthy? They're healthful. If they
were healthy, wouldn't they still be alive?

The Answer:

There are those who make a distinction between healthful, meaning "conducive
to health; wholesome or salutary:

a healthful
diet

" and healthy, meaning "possessing or
enjoying good health or a sound and vigorous mentality:

a
healthy body; a healthy mind

." Most people use "healthy"
for both meanings. Merriam-Webster's Dictionary of English
Usage notes that this distinction was invented in 1881
(several centuries after both words first appeared), and that "if you
observe the distinction between healthful and
healthy you are absolutely correct, and in the
minority." On the other hand, "If you ignore the distinction you are
absolutely correct, and in the majority."